In general, the MEMS inertia sensor, which is widely used, comprises a weight (movable member) and a support beam (elastically deformable member). An acceleration sensor is the device in which the weight is supported by the support beam movable along a certain linear axis defined relative to the substrate on which such an MEMS inertia sensor is formed and in which the displacement of the weight due to the acceleration exerted on to the substrate is converted to a corresponding electric signal by means of an LSI circuit. On a substrate where an MEMS inertia sensor is formed, a weight is supported by a support beam movable along both a first axis and a second axis defined perpendicular to each other; the substrate as a whole is rotated about a third axis perpendicular to the substrate while the weight is being vibrated along the first axis by means of a vibration generating unit; and then the weight vibrating along the first axis comes to move in the direction of the second axis due to the Coriolis force generated due to the rotational force. The angular velocity sensor is that which converts the displacement of the weight to an electric signal by means of an LSI circuit.
These inertia sensors have a common feature that a weight serves as a sensing element of the sensor. Since this weight is a mechanical element, it may be displaced even when acceleration other than a signal to be measured is exerted thereto. And such an unwanted displacement of the weight may be converted to an electric signal by the LSI circuit so that the resulted electric signal becomes noise, which deteriorates the measurement precision of the inertia sensor. In another case where such an electric signal has a magnitude that is beyond the measurable range of the LSI circuit, that is, where the LSI circuit saturates, the electric signal may cause a dysfunction of the inertia sensor as a result of the signal to be detected being mixed into the saturated signal.
In order to suppress such an erroneous operation of the inertia sensor due to acceleration other than a signal to be measured and such a dysfunction thereof due to an erroneous output, it suffices to prevent the detecting element from mechanically responding to acceleration other than a signal to be measured. Accordingly, it is desired that a vibration-proof structure is fabricated in such a manner that vibration-proof parts for preventing the acceleration other than a signal to be measured from being transmitted to the inertia sensor via the substrate on which the inertia sensor is mounted should be interposed between the inertia sensor and the substrate on which the inertia sensor is mounted.
The vibration transmissibility of the vibration-proof structure is given by Tr (%) of the formula (1) in FIG. 17. Therefore, in order to decrease the value of the vibration transmissibility Tr (%) of the vibration-proof structure for the purpose of suppressing the transmission of acceleration other than a signal to be measured to the detecting element of the inertia sensor, it is necessary to lower the characteristic frequency of vibration f0 (Hz) of the vibration-proof structure provided between the inertia sensor and the substrate on which the inertia sensor is mounted.
It is to be noted here that the characteristic frequency of vibration f0 (Hz) is given by the formula (2) in FIG. 17. Accordingly, in order to lower the characteristic frequency of vibration f0 (Hz) of the vibration-proof structure in the sensor wherein the sensor substrate as a sensing member is integrally mounted in the packaging member, it is primarily considered to increase the mass m (kg) of the vibration-proof structure as a whole. However, in the sensor wherein the sensor substrate as a sensing member is integrally mounted on the packaging member, there is a limitation of space within the packaging member, and the increase in the dimensions of the substrate constituting the sensing member of the sensor amounts to the increase in the dimensions of the sensor as a whole. This is not desirable from the standpoint of production cost or usability of the sensor. Further, if the material of the substrate serving as the detecting element of the sensor is to be substituted for another material having larger density, the process of manufacturing the sensor must be changed. This is undesirable since the time required for development is prolonged, causing the increase in cost. It is therefore difficult to employ the method of increasing the mass of the vibration-proof structure.
An alternative method may be considered that decreases the modulus of elasticity k (N/m) of vibration-proof structure so that the characteristic frequency of vibration f0 (Hz) of the vibration-proof structure given by the formula (2) can be lowered in the sensor wherein the sensor substrate as a sensing member is integrally mounted on the packaging member. Concerning this method, the following techniques have been proposed.